The term planet is ancient, with ties to history, science, mythology, and religion. Several planets in the Solar System can be seen with the naked eye. These were regarded by many early cultures as divine, or as emissaries of deities. As scientific knowledge advanced, human perception of the planets changed, incorporating a number of disparate objects. In 2006, the International Astronomical Union (IAU) officially adopted a resolution defining planets within the Solar System. This definition is controversial because it excludes many objects of planetary mass based on where or what they orbit. Although eight of the planetary bodies discovered before 1950 remain "planets" under the modern definition, some celestial bodies, such as Ceres, Pallas, Juno and Vesta (each an object in the solar asteroid belt), and Pluto (the first trans-Neptunian object discovered), that were once considered planets by the scientific community, are no longer viewed as such.

Planet (locomotive)

History

The ninth locomotive built for the L&MR, it was Stephenson's next major design change after the Rocket. It was the first locomotive to employ inside cylinders, and subsequently the 2-2-0 type became known as Planets. On 23 November 1830 No.9 Planet ran the approximately 50km from Liverpool to Manchester in one hour.

Planet-type locomotives

Six further of the type were ordered by the L&MR from Robert Stephenson & Co. Three more were supplied by Murray & Wood in Leeds, to whom Robert Stephenson & Co. had sent the drawings for their manufacture.

The Planet (from 1830) and the Patentee (from 1833, also designed by Stephenson) were the first locomotive types to be built in large numbers.

Improvements

The Planet locomotives appear closer to subsequent types, and conversely look quite different from Rocket although only a year separated these two designs (Stephenson's Northumbrian representing an intermediate evolutionary step).